Martin Eigel, Martin Heida, Manuel Landstorfer
01.05.2021 − 30.04.2024
Electrochemical electricity storage is a central pillar for a large variety of industrial goods, ranging from power sources for medical devices to electric vehicles and large scale battery plants. In 2019 this was honored by the Nobel price in chemistry, awarded “for the development of lithium-ion batteries (LIBs)”. The central innovation is the concept of intercalation, the physico-chemical process by which a lithium ion is stored within some solid host material. This process is essential for the safety, durability and energy density of modern LIBs.
However, all current and future LIBs face a common issue: they degrade in their lifetime upon usage. This degradation is in general a superposition of various ageing effects and depends on external (time dependent) parameters, e.g. the rate at which a battery is charged and discharged. Quantitative and qualitative knowledge of the degradation is of ultimate importance to estimate the lifespan of a battery, set up control engineering and ensure safety.
The project aims at developing a data-driven methodology to recover the dynamics of battery ageing on the basis of a parametrized mathematical model and experimental data. We want to determine the evolution of certain parameters of the model as function of the cycling number N. This is to be achieved by setting up a two time-scale PDE model, where the small time scale covers one charge/discharge cycle and the large time scale the number of such cycles.
To solve this statistical inverse problem for the degrading parameters, we will use recent ideas from invertible neural networks . Moreover, low-rank surrogate models for parametric PDE solutions will be employed in order to efficiently cope with the high model complexity [1,3].
In the long term, our approach can lead to real-time tracking and estimation of the battery health, which is of paramount importance for electric mobility. It can help to determine the residual value of an aged battery as well as its future lifetime, which is crucial for stationary energy storage devices and thus the “Energiewende”. The solution of statistical inverse problems with machine learning techniques is still in its infancy. We hope that we will contribute to the understanding of how to use Deep Neural Networks for these common problems, which will then be applicable in many fields.
Valid models of the physics in batteries are crucial in understanding the ageing mechanism and in developing new concepts to counteract the ageing. Landstorfer et al.  presented a modeling framework for simulations of parametrized LIBs based on homogenization in space, assuming only one active particle in a periodic unit cell, which is further assumed to be spherical.We extended the model to multiple active particles in one unit cell. A preprint is being written as of now.
Battery ageing is basically a two-timescale problem. Analogously to the space homogenization, we are deriving a homogenization in time. Currently, we are in the process of finding suitable scalings of smallness parameters and exploring the respective results. In particular, we consider two kinds of coupled problems: One is a diffusion problem, and the other one is a reaction problem. To tackle this problem, we are applying methods of asymptotic expansions to find possible homogenized problems and prove convergence using two-scale convergence theorems, among others.
du/dt = -∇ ⋅ ( D ∇u)
dD/dt = d(D, u, du/dt, ∇u)
Plus boundary/initial conditions
du/dt = -∇ ⋅ ( D(u,v) ∇u)
dv/dt = R(u,v)
Plus boundary/initial conditions
The degradation of a battery with each cycle is caused by many different phenomena. One cause of degradation is the cycle dependence of the diffusion coefficient D appearing in the PDE-model . As a simplification, the time-dependent behavior is assumed to satisfy a simple evolution equation dD/dt = g(D(t)), which is an ODE in D with a right-hand side g.
Using a data-driven approach, we succesfully retrieved the rhs g based on charge-discharge-curves of batteries (which may be measurement data, or solutions of the forward problem in ). Since this is an inverse problem, we used Bayesian Inference. In particular, we applied Markov-Chain Monte-Carlo (MCMC) to sample from the posterior and additionally calculate the Maximum a-Posteriori (MAP) directly in cases where we are not interested in the uncertainty. A preprint is currently being written.
In battery ageing, multiple ageing effects can and do occur, for example a degradation of the reactivity as one effect, and a degradation of the diffusivity as another one. Depending on how the battery is used during its lifetime, different effects have a different impact on the ageing. Given the charge-discharge curves of a used battery, our goal is to identify which effect occurred with what impact. This way, we can make reliable predictions of the remaining lifetime of the battery.
The user behavior can be modeled as a curve, mapping the time time into an L-dimensional parameter space, where L is the number of occurring effects. This curve is parametrized with K weights, which represent the weights of the ageing effects. It is K ≥ L, since one effect progression can be modeled as a function of the time with more than one parameter.
The curve is then further mapped into an L-dimensional submanifold of an observation space X, which corresponds to the charge-discharge curves. Note that X may or may not be infinite dimensional. Given these observations, we successfully recovered the degradation weights of different ageing effects, both in a well-defined, invertible case, as well as in the case of an inverse problem. In the latter case, we again applied Bayesian Inference. A preprint is currently being written.
Time and length scales of a typical battery system. The scale transition is carried out with asymptotic and homogenization methods.
The microstructure of a typical battery electrode consists of intercalation particles (blue), which are surrounded by a liquid electrolyte through which lithium ions travel an electric current (red arrows). For the purpose of computer simulations, digital representations of such microstructures are required, which can be obtained with proper meshing techniques .
Upon repetitive charging and discharging, called cycling, the capacity of a battery decreases. This originates from various superimposed ageing phenomena, for instance due to microscopic crack formations within a particle.
The different ageing mechanisms can be expressed mathematically in terms of cycle number N dependent parameters. Microscopic cracks within particles yield a cycle number dependent diffusion coefficient D=D(N), while a degradation of the solid electrolyte interphase (SEI) yields a degradation of the intercalation reaction rate L=L(N). These yield, by numerical simulations, qualitatively different spectra of the cell voltage, which can be exploited to determined as an inverse problem the specific ageing effect in a real battery system.